The invention relates to a tensioning device for extending a threaded bolt by way of tension on the threaded end section thereof, having a supporting tube surrounding the threaded end section, a cylinder which is arranged in continuation of the supporting tube and has at least one piston axially movable therein, an exchange socket which is screwable to the threaded end section and is configured such that it can be carried along axially by the piston, and a pin which is arranged in an axially movable manner in a longitudinal guide in the exchange socket and the pin end of which is axially supportable against that end of the threaded bolt that has the threaded end section.
A hydraulically operating bolt tensioning device having these features is known from US 2011/0271798 A1. The tensioning device has measures for monitoring the thread projection, i. e. the length of the threaded end section, gripped by the exchange socket, of the threaded bolt. To this end, the exchange socket is provided with a longitudinal channel in which a measuring rod is fitted in a longitudinally movable manner. The lower end of said measuring rod is supported axially on the end face of the threaded bolt to be tensioned. At its other end, the measuring rod is provided with coloured markings, by way of which it is possible to read, with respect to the exchange socket, whether the thread projection, i. e. the length of the threaded section gripped by the exchange socket, is large enough for the tensioning process. Although the measuring rod according to US 2011/0271798 A1 allows sufficiently precise assessment in many cases, the assessment of the actually used thread projection is imprecise in cases in which the end face of the bolt is not closed, but rather has, for example, a central thread depression in which the lower end of the rod is then supported.
The invention has the object of enabling assessment of the actual thread engagement even in cases in which the known tensioning device provides imprecise results.